Two decades after the first terrorist attack on the World Trade Center, survivors and families of the six people killed -- two of them Long Islanders -- gathered at Ground Zero Tuesday to commemorate their loss.

They were joined at the Sept. 11 memorial by first responders and dignitaries, including Mayor Michael Bloomberg, for a short, somber ceremony marking the moment on Feb. 26, 1993, when a truck bomb blew up in a parking garage under the north tower.

For Stephen Knapp, 38, of Staten Island, who lost his father in the bombing, standing at the site of the World Trade Center on Tuesday flooded him with memories.

"With him working here and dying here . . . it always takes me back to being a little kid," said Knapp, who was 18 and in school when his father, Stephen Knapp, who was chief mechanical supervisor for the World Trade Center, died.

At the memorial, where the north tower once stood, the peal of a silver bell rang out at 12:18 p.m., the time of the explosion.

Among the six people killed were two Long Islanders: John DiGiovanni, 45, of Valley Stream, who had been visiting the building; and Monica Rodriguez Smith, 35, of Seaford, a pregnant secretary who was working her last day before maternity leave.

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Their names, along with those of the four others who died -- Stephen Knapp, Robert Kirkpatrick, William Mercado and William Macko -- were read aloud at the ceremony by Knapp and Michael Macko, whose father perished in the blast.

Radical cleric Sheik Omar Abdel-Rahman and several followers were convicted of trying to blow up the World Trade Center and other landmarks amid a holy war against the United States. Abdel-Rahman, who is blind, is serving a life prison term under restrictive conditions designed to prevent him from communicating with followers.

The ceremony also was attended by David Dinkins, mayor at the time of the attack.

, Knapp brought along his own children, ages 4 and 6. While they were still too young to appreciate what happened, Knapp said he plans to continue teaching them about it.

"Unfortunately, it's going to be a lesson to them about what this world can be like and the horrors in it. But we can let them know their grandfather's death wound up helping thousands of people in the future," Knapp said, referring to the evacuation plans for the World Trade Center that were revamped after the 1993 bombing. "On Sept. 11, they were saved because of what happened to him. It shows what good can happen as well."